<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
    <style>
        .box {
            width: 1000px;
            border: 1px solid black;
            margin: 0 auto;
        }
        .l {
            float: left;
        }
        .r {
            float: right;
        }
        .clearfix:after,.clearfix:before{
            display: block;
            content: '';
        }
        .clearfix:after {
            clear: both;
        }
        .clearfix{
            zoom: 1;
        }
    </style>
</head>
<body>
<div class="box clearfix">
    <div class="l">aaaaaaaa</div>
    <div class="r">bbbb<br>bbbb</div>

</div>
</body>
</html>